Typical elevator systems include a car and a counterweight moving along guiderails in a vertical elevator shaft. The car and the counterweight are connected to each other by hoist ropes. The hoist ropes are wrapped around a sheave located in a machine room at the top or bottom of the elevator shaft. The sheave can be moved by an electrical motor, or the counterweight can be powered by a linear motor.
Rope sway refers to oscillation of the hoist and/or compensation ropes in the elevator shaft. The oscillation can be a significant problem in a roped elevator system. The oscillation can be caused, for example, by vibration due to wind induced building deflection and/or the vibration of the ropes during operation of the elevator system. If the frequency of the vibrations approaches or enters a natural harmonic of the ropes, then the oscillations can be greater than the displacements. In such situations, the ropes can tangle with other equipment in the elevator shaft, or come out of the grooves of the sheaves. If the elevator system uses multiple ropes and the ropes oscillate out of phase with one another, then the ropes can become tangled with each other and the elevator system may be damaged.
Various methods control the sway of the elevator rope by applying tension to the rope. However, the conventional methods use a constant control action to reduce the rope sway. For example, the method described in U.S. Pat. No. 5,861,084 minimizes horizontal vibration of elevator compensation ropes by applying a constant tension on the rope after the vibration of the rope is detected. However, applying a constant tension to the rope is suboptimal, because the constant tension can cause unnecessary stress to the ropes.
Another method, described in U.S. Patent Publication 2009/0229922 A1, is based on a servo-actuator that moves the sheave to shift the natural frequency of the compensation ropes to avoid the resonance of the compensation ropes with the natural frequency of the building. The servo-actuator is controlled by feedback that uses the velocity of the rope vibration at the extremity of the rope. However, that method only solves the problem of compensation rope vibration sway damping. Furthermore, that method necessitates the measurement of the ropes sway velocity at the extremity of the rope, which is difficult in practical applications.
The method described in U.S. Pat. No. 7,793,763 minimizes vibration of the main ropes of an elevator system using a passive damper mounted on the top of the car. The damper is connected to the car and the rope. Distances and a value of the damping coefficient of the damper are used to reduce the rope sway. However, in that method, the number of dampers is proportional to the number of ropes that are controlled. Furthermore, each damper is passive and engages continuously with the rope, which can induce unnecessary extra stress on the ropes.
Other methods, see, e.g., U.S. Pat. No. 4,460,065 and U.S. Pat. No. 5,509,503, use purely mechanical solutions to limit the sway amplitude by physically limiting the lateral motion of the rope. Those types of solutions can be costly to install and maintain.
Accordingly, there is a need to optimally reduce the sway of the elevator rope.